


Cagey

by CleverCatchphrase



Series: Extra Explicits [3]
Category: Deltarune (Video Game), Undertale (Video Game)
Genre: (I wrote this to get off so don't expect virtue in this fic), (singing: ''since we're not related it'll be okay''), Chastity Cage, Chastity Device, Does this count as a high school/college AU if those places are only mentioned?, M/M, Sibling Incest, Size Difference, elements of toxic relationships, gender neutral kris with traditional male anatomy, non-verbal kris
Language: English
Status: Completed
Published: 2021-01-03
Updated: 2021-01-03
Packaged: 2021-03-13 15:41:34
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,883
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/28530885
Author URL: https://archiveofourown.org/users/CleverCatchphrase/pseuds/CleverCatchphrase
Summary: Asriel's finally come home from college for Thanksgiving break. Kris has been waiting a long time.
Relationships: Asriel Dreemurr & Kris (Deltarune), Asriel Dreemurr/Kris (Deltarune)
Series: Extra Explicits [3]
Series URL: https://archiveofourown.org/series/1193671
Kudos: 10





	Cagey

**Author's Note:**

> Me: *sees fan art of hyperdeath Asriel in a chastity cage*  
> Me:...  
> Me:...  
> Me:...  
> Me:...That's kinda hot.
> 
> Also me: For shits and giggles, I wonder how many smut fics I can find of Kris and Asriel.  
> Me: Oh? Only one? LET ME FIX THAT.

Kris grabbed their school uniform from their gym locker and made a hasty retreat to the bathroom. The monsters shifted to the side to let them pass, not thinking much of it as they changed into their own clothes, except for one who stared, transfixed at the corner where the human had disappeared.

“Hey, you guys ever noticed how Kris always goes into the restroom to change?” Snow asked in a hushed tone to the other guys. “I wonder why they’re so secretive.”

“Maybe they got an embarrassing birth mark on their body!” Jockington speculated. “Or a wicked-ugly tattoo!”

“I doubt they have any tattoos,” Berdly said in his snobbish, know-it-all tone. “Kris just turned eighteen- which is the minimum age you can get a tattoo, by the way- this past summer, but this behavior began back when high school started. _I_ suspect it is because they feel inadequate in the presence of magical monster bodies, and thus hide themselves away out of shame!”

“Nah, Kris doesn’t care about that,” MK said, dropping into their conversation. “They’re close friends with plenty of monsters who can get away with full nudity and that doesn’t make them uncomfortable. But something about them _has_ changed. They weren’t always this private about their body. I remember plenty of times Kris would change with us last semester, even though they had their weird days then, too. It’s just gotten worse lately.”

“Maybe it’s a puberty thing?” Snowy suggested. “Do humans go through some kind of gross metamorphosis like frogs?”

“Possibly,” said Jockington while MK shrugged. 

“That would explain why they never use the community shower after gym class,” Berdly mumbled. “You guys are lucky you don’t have to sit behind them in fifth period. They positively reek after circuit days!”

The monsters whispered back and forth for a minute more, but their gossip came to an abrupt stop when Kris returned to put their gym clothes back in their locker.

The monsters stared, dead silent like a herd of deer trapped in the blinding headlights of a semi-truck bearing down on them before Kris tentatively waved their hand in an awkward greeting, and left the locker room.

“Maybe… maybe Kris is just weird,” one of the teens suggested once they were sure they were gone, and began to file out themselves. “Whatever the reason, it’s their business and their business only.”

“Yeah,”  
“I guess,”  
“Whatever.”

From behind the last row of lockers, Kris smirked with voyeuristic giddy, hidden from their line of sight. If only the other students knew of the perverted secret they had that was hidden just beneath two thin layers of clothing- a secret they had been carrying with them for over three months now.

Their guesses, while plausible, were way off mark, but that was fine. That meant Kris had done well not to raise suspicion. They had been careful for so long and planned to keep it that way, because if they got caught- well… what possible excuse could they give to the teacher, the principal, to _anyone_ who discovered they were wearing a chastity belt in school?

Kris walked down the hall to their next class, back straight, eyes forward, and face as neutral as possible. Gym class was always the hardest period to get through when they were caged like this. One glace from roaming eyes when their pants were down, and their secret could be blown right out of the water. But Kris would have been lying if they said the thought of being caught didn’t turn them on.

At every start of class when they did their warm ups, Kris wondered if the student behind them could see the gleam of the stainless steel peak through their briefs and short gym shorts when they bent over to touch their toes. It baffled them how no one could hear the clinking of the padlock bouncing off the metal bars when they did jumping jacks, or notice the ridged creases in the fabric at their crotch when a classmate held their feet down to do sit ups.

With bodies so close and so much physical contact, it was amazing no one noticed _anything_ these last twelve weeks. Or- if someone _had_ noticed and just hadn’t said anything- then it was quite a delicious surprise to learn that other degenerate sadomasochist existed in the monster world and were polite enough to keep their mouths shut.

Arriving at their next class, Kris promptly took their seat and exhaled in relief. Despite all their research telling them exercise helped get rid of unwanted boners, the thrill of being an accidental exhibitionist in P.E. never failed to give them one.

Kris shifted uncomfortable in their chair, the swollen flesh of their cock straining against the cruel metal bars that refused to let them get erect, and the almost-choking sensation of the cock ring around their scrotum as their balls drew themselves up in arousal.

As discreetly as possible, Kris rocked forward and back. Even through two layers of clothing, the sensation of the steel grating against the hard plastic of their chair was enough to send shivers through their shaft. It wasn’t anywhere near as satisfying as masturbation, but it was better than nothing.

Sighing in frustrated boredom, Kris let their eyes wander around the room, ignoring their teacher’s lesson. They settled on the clock, counting down the minutes until the school day was done. Closing their eyes, they slipped into daydream, fantasizing about the one they loved most, and wondering how they were doing so far away. 

In a few more hours, their torture would come to an end, because tonight, for the first time in exactly one hundred days, Kris’s key holder was coming home.

* * *

Toriel fretted about the house like a humming bird with OCD, straightening the furniture, dusting the trinkets, and adjusting the flatware on the table once, twice, three times over. Kris sat in the oversized armchair facing the TV, but was watching her bustle about from the corner of their eye. After all, what could there be on TV that was more entertaining than this?

“Oh, isn’t it exciting, Kris?” Toriel asked rhetorically, just to fill the air with conversation. “Asriel’s finally getting to come home from college! I wonder if he’s gotten any taller since the semester started? I do hope he’s been eating right. Oh, how I wish there were more school holidays between Labor Day and Thanksgiving! Three months is just too long to be apart! Don’t you think so?”

Kris bit back a snort. She didn’t know the half of it. Though they did agree it had been far too long since they last saw Asriel. It was unfair his university was out of state. It made visiting outside of holidays a hassle and not worth the effort. The last time they and their brother had been together had been the night before his four-hour drive to university to start his sophomore year, and only a few days before Kris had started their senior year of high school.

It had also been the night Asriel had confiscated Kris’s right to indulge in sensual pleasures on their own, locking them in the cage and keeping the key for himself, forcing them into a one hundred day commitment to celibacy. 

“ _I love you so much, Kris,_ ” he had whispered in their ear as rough paw pads lined with silken fur passionately traced their ribs and abs. Kris remembered the night well, even in their lust-drunk fervor; from the heat of his breath on their bare neck, to the musky scent of his arousal, to the pale green glow of the plastic stars on the ceiling overhead that seemed to spin and swirl on their own accord. 

“ _I love you more than anyone else ever could. More than anyone else ever will. You are_ mine _and mine alone. Now and forever._ ” 

Kris would have returned the sentiment if they could- letting him know the desire to for possession and to be possessed went both ways, but words were never their forte, and the ball gag in their mouth hadn’t helped.

“ _You belong to_ me. _Only I can have you. Only I can touch you,_ ” Asriel had said in that rich, hypnotic voice of his that could leave Kris _begging_ for him to claim them exclusively.

“ _And just to make sure no one else can tamper with what’s mine, I want you to wear this until the next time I come home._ ”

The chastity cage was by no means a new toy. It had been one of the first things the siblings had experimented with back when they both realized their feelings for each other were stronger than just familial or plutonic love. But to wear it for a length of time longer than a few days was uncharted territory for the both of them.

Could Kris even do it? Their particular cage had been designed with an opening at the tip where the glands would reach, so urinating wouldn’t be a problem, but what about bathing? What about morning wood? What about the swim unit at school?

Asriel had clicked the lock into place before Kris could voice a single concern. Blinking up, they watched him spin the key around his finger before tossing it onto his suitcase on the floor, and resumed their previous carnal activity.

Kris was all too happy to submit, shrugging aside their worries to savor the pleasure of the moment, knowing it would be their last for a while, and trusting that they were in good hands.

A knock on the door drags Kris out of the memory, and not a moment too soon. Just reminiscing about that night was enough to get them horny, and even though their cage would prevent any tent their cock attempted to pitch, it would do nothing if Kris started to drip. Best to leave the recollection in the past for now.

“He’s here!” Toriel giggled with glee as she rushed to the door. Kris got up and stood a few paces behind her. As badly as they wanted to get their hands on their brother, they would let her have him first, if only to keep up pretenses. 

“Asriel!” Toriel exclaimed, throwing her arms around her son when he’s barely even a foot inside the door. Kris watched form a few steps back. It was more of a struggle to keep their jealously in check than they realized.

“How was your drive? Do you need help unloading your things? Oh, just look at how much your mane has grown! You’re getting more handsome by the day!”

Asriel humored her, politely making small talk while simultaneously trying to inch past the threshold. He had a duffel bag in one hand, a rolling suit case in the other, and a back pack stuffed to the brim across his shoulders. He glanced up and spied Kris waiting, and his innocuous smile gained a salacious gleam.

“Actually, Mom, my drive did tucker me out a bit. Would you help me get my stuff to my room?”

“Of course, honey!” Toriel said without question, and took off with his bags upstairs without a second thought.

As soon as she was out of sight, Kris seized their chance and tackled into Asriel for a hug of their own.

“Oof! Ha ha, I wonder if you missed me?” Asriel teased, recovering from the impact. Kris buried their face into his shirt- their head only coming up to the bottom of his breastbone. There weren’t enough words to describe how much they missed him. From his down-soft fur to his solid mass, to those muscled arms that would have no trouble restraining him to the floor or the wall yet were gentle enough to cradle them like fragile porcelain. And, of course, they couldn’t forget about the monster that lived between the legs of their monster brother.

“That was very polite of you to be so patient with Mom until she was out of the room,” Asriel praised, then dropped his voice to a whisper. “Have you been this good the rest of the time I was gone?”

Smirking into his shirt, knowing he couldn’t see their sneaky grin, Kris shifted their arms down from the small of their brother’s back to grope his ass, making Asriel flinch in surprise.

“Cheeky bastard,” he said, growling with want and longing. But before he could make a move of his own, Toriel returned to the den.

“I left your things beside your bed,” she informed him. “I figured you would want to unpack your own belongings rather than have me rifling through them.”

“Thanks, Mom,” Asriel said in his plucky Boy Scout voice- a perfectly innocent tone that gave away nothing of his ulterior motives. 

Feeling particularly naughty, Kris gave their brother’s ass another squeeze, making him tense up in an effort not to squirm, but Asriel was too well practiced to let such a simple molestation crack his façade. 

“Four hours is a really long time to be driving. Are you hungry? Can I make you a snack or refreshment?”

“Nah, I’m good!” Asriel said, chipper as ever. He patted Kris’s shoulder. “I’m a little tired, but I really want to catch up with Kris and hear how high school is coming along. I really missed them.”

“He he! From the looks of it, they missed you too!” Toriel agreed, noting how Kris was still hugging him from her perspective.

“Yeah, no ki- _i_ -dding!” Asriel squeaked, voice hitching as Kris made yet another daring squeeze. At this, he finally forced them apart as gently as possible.

“C’mon, Kris! Let’s go upstairs to our room. I want to know _everything_ about your senior year!”

Kris nodded excitedly, impressed by Asriel’s ability to keep such a straight face under torment, and talent to flip from a seductive tone to a Christian one at the drop of a hat.

“Alright, kids! I’ll call you down when dinner is ready, okay?”

“Thanks Mom!” Asriel called back from half way up the stairs, one of Kris’s wrists clutched tight in his fist as he practically dragged them along behind him.

The urgency in his grip didn’t subside until the bedroom door was securely closed behind them, where the two then collided together like neodymium magnets, their desire just as electric. Alone at last, they threw all their restraint out the window along with half of their clothes.

“Brazen little shit,” Asriel panted between kisses when they both came up to breathe. “I have half a mind to tie you up and fuck you senseless for that risky stunt you pulled in front of Mom, but that’s exactly what you want, isn’t it?”

Kris answered by forcing his lips back onto theirs, and dragging their nails down his back.

“Goddam tease,” Asriel moaned. He brought his leg up to grind against Kris’s groin and felt the hard metal of their cage against his thigh. His eyebrows arched and he hummed approvingly.

“Still confined I see. You _have_ been good while I was away.”

Grabbing them by the thighs, Asriel lifted Kris with ease and carried them over to the bed. “Let’s unwrap the rest of this package,” he said, a hungry glint in his eyes. “I need to inspect my goods and make sure nothing is damaged.”

Kris was all too happy to lose the rest of their clothes. Their shirt had been torn off the minute they stepped into the bedroom, and they had practically clawed off Asriel’s pants themselves, but they had another surprise for their brother beneath their briefs and were all too ready to share.

Kris looped their arms around their brother’s neck, handing from him as he removed their pants at an agonizingly slow rate- punishment for earlier, they knew.

“Have you managed to get an erection while I was gone?” Asriel asked. Kris shook their head ‘no’, a serene smile on their face.

“Good. Have you tried to touch yourself at all these past one hundred days?”

Kris shook their head again, more insistent this time, anticipation making their patience short. 

“Good. Very good…” Asriel cooed, and slid off their underwear to expose their incarcerated cock. “Lean back,” he ordered, and with the utmost tenderness, Asriel reclined Kris onto his pillows. He eyed them up and down for a moment, licking his upper canines seductively.

“Oh? What’s this?” Asriel asked, catching sight of something protruding form Kris’s ass. With delicate fingers, he gently removed the object to reveal a large, stainless steel butt plug. He clicked his tongue in displeasure.

“Kris, I can’t believe you would let me catch you using one of these,” he said reproachfully. “You know I like it more when you’re super tight for me.”

Kris bit their inner cheek and creased their brow. It was all they could do not to sneer. As if an asshole stretched two inches would feel like much of a difference to him. Kris needed to take all the preparation they could get if they ever wanted to survive their monster brother’s monster cock. Being twice their size, his dick was as long and as thick as their forearm, and it felt as if their whole body was being torn asunder every time they fucked; so much so that they wondered if one day Asriel’s cock would actually split them in two if he wasn’t careful.

“You understand you’ll need to be punished for this, right?” Asriel murmured, removing their arms form his neck to pin them above their head. “Your cage stays on until I’ve had my way with you, and you’d better not cum before I’m done. Got it?”

Kris swallowed, nervous and unsure if they could do it. The last time Kris had been fucked while caged had been the night before Asriel left. They had orgasmed then, unable to help it, and cumming while confined had felt borderline unbearable. They’d managed not to climax since, but that had also only made them more pent-up. Besides, not touching yourself was way different than being penetrated by a two foot long, giant goat dick. Would they even last the first five seconds?

All of Kris’s concerns evaporated like dew when Asriel opens his mouth and drags his long, wet tongue up from their hip to collar bone and across their throat. His mouth hovers at his neck a while, lightly biting the skin on their shoulder, enough to leave raised red marks, but not to break skin or draw blood. Still, Kris hissed through clenched teeth- the sensation of being repeatedly pinched was not a pleasant one.

Despite their best efforts to grin and bear it, they struggle and squirm beneath him, curling their back in an effort to get some space, and unknowingly fall into a trap.

Seizing his chance, Asriel impales Kris when they unwittingly present themselves to him. His cock, rock solid and slick with precum, plunges into his sibling with ease, cutting off all attempts of complaint with a choked gasp. Their body practically inverts beneath him, flipping from concave to convex as they arch their spine and press themselves against him, stomach to stomach.

“Breathe,” Asriel commands, slowly drawing out of them, and Kris takes a shuddering breath, gulping for air. “Good job,” he praised, planting kisses on their cheeks as he eased his way back in. “Keep remembering to take deep breaths and the pain with pass. You’re out of practice. But I guess I’m the only one to blame for that.”

He set a rhythm, slow but unrelenting, forcing Kris to take him in his entirety from head to hilt, before almost completely removing himself only to force his way back in again.

“So how _has_ school been for you?” Asriel asked while absentmindedly humping their sibling. “Are you enjoying your senior year? It’s almost halfway over now, isn’t it?”

Kris choked back a whimper and bit their tongue to keep quiet. They couldn’t let their mom hear. They screwed their eyes shut.

“You don’t say,” Asriel snorted sarcastically. “College has been pretty okay for me. I can’t get you out of my head, though. I’ve been thinking about this day ever since my first night back in the dorms. You can’t even imagine how many times I thought about skipping class for a whole day just to drive back down here and _have_ you. I’ve missed you _so much_.”

Using one massive paw to keep Kris’s arms held above their head, Asriel glided his other paw down their front, stopping at their crotch to fondle their balls. Their dick, already soaking Asriel’s fur from so much leaked precum, was swollen and angry, straining against the bars of their chastity cage in a futile effort to break free.

“My, my, look how wound up you are!” Asriel teased. “Your nuts are so swollen, they barely fit into my palm!” He gave a light squeeze, eliciting a desperate cry from Kris’s lips and causing their hips to buck involuntarily. 

“Ah, ah! Remember, you can’t cum until after I do, and I’ve still got a long way to go.”

Head bowed, Kris nodded weakly, and let Asriel resume. Hooking their ankles behind his thighs, they draw him closer, almost pleading, and Asriel, wanting Kris just as bad as Kris wanted him, finally complied.

He sped up his pace, pumping in and out with increasing velocity until his movements became sloppy and uncoordinated- mind lost in the primal urge to inseminate and breed. At some point he let go of Kris’s wrist to grab both sides of their waist and fuck them harder, and Kris dug their fingers into his arm fur, anchoring themselves in place until Asriel drove himself to climax.

The pair held their breath until he finished his orgasm- making any nose akin to pain was dangerous when the house’s other residents weren’t away or asleep.

Kris came back from the high first, blissfully sighing as the sensation of hot cum gushed into them, filling up their body with delicious warmth. Asriel came down a minute later, gasping from exertion and blinking back to reality. Carefully he pulled out, a small stream of semen dripping from Kris’s ass in his wake.

Kris flashed him a beautiful, disheveled smile, and he smiled back, tired and spent, before shifting his gaze back down. Kris’s cock, practically purple within the cage had managed not to cum.

“You did it!” Asriel barked a laugh, genuinely beside himself, before covering it with a cough to keep up his domineering persona. “Well, a deal’s a deal,” he purred. “Time for your reward.”

Reaching into the long, golden fur of his mane, Asriel pulled out a key that hung from a thin chain around his neck.

“I’ve worn this every day since I locked you up,” he said, eyebrows wagging. “I made myself promise not to take it off if you couldn’t take yours off.”

Kris sat up slightly as Asriel slipped the necklace off past his horns.

Cradling their dick in one paw, he unlocked the cage and removed it with his free hand. The relief of the pressure alone was enough to make Kris moan in ecstasy, but being a good brother, Asriel was kind enough to gently message some life back into the cramped flesh.

“Heh, it’s like watching a desiccated sponge soak of water,” Asriel said, marveling at how Kris’s cock seemed to grow exponentially as it expanded until it filled his palm. A few seconds more and it was standing erect on its own, pulsing with a need for attention.

“God, I think being locked up for three months has made you even bigger,” Asriel remarked, eyeing Kris’s dick hungrily. “But it’s not good to let fuel just sit around that long. We gotta _flush the tanks_.”

Kris scooted back against the pillows to lean on the headboard and spread their legs wide as Asriel shuffled down.

“I learned a few new tricks while at college,” he boasted as he positioned his head over their cock. “I wonder how long you can last against this?”

Opening his maw, Asriel let his long tongue out once again. With expert control, he coiled it around their shaft, flicking the slit a few times before engulfing the entire length in his cheeks.

Kris grunted, grabbing onto their brother’s horns for balance as Asriel began bobbing up and down. In all honesty, they couldn’t tell if this oral was any better or worse than blowjobs of past. Simply being touched for the first time in three months was enough to drive them wild. The hot, wet vacuum of Asriel’s mouth was heavenly compared to the hard, restricting steel, and they couldn’t bear a millimeter of their dick being left out.

Yet again Asriel had to resort to holding Kris in place when they began to thrust. With one paw firmly planted on their stomach, Asriel used the other to tease their sack, squeezing and rubbing the delicate organs within the soft skin, driving Kris absolutely wild.

Uneven, ragged breathing was Kris’s tell, and Asriel clamped down as hard as he dared on his sibling’s sensitive shaft, sucking like mad as Kris came by the bucket load.

Asriel’s mind drifted as his cheeks filled and swallowed, filled and swallowed. Did all humans cum this much like Kris? Did every human taste as sweet? Or were both of these the side effects of a monster upbringing and exposure to so much magic?

Asriel hadn’t become close enough to any other humans yet to find out, but there was a part of him that didn’t really want to. Kris alone was more than enough. There were all he ever wanted. They were all he ever needed, and it was comforting to know that Kris felt the same.

When the last bit of cum was expelled and Kris’s dick stopped twitching, Kris let out an exhale of exhaustion and flopped back, almost visibly deflating on the bed.

Asriel sat up, licked his lips clean of any semen he might have missed, and shuffled up beside their sibling to snuggle them in their shared afterglow.

“A little more than half a year and you’ll be off to college, too,” Asriel said, striking up a conversation to chase away the quiet. “Have you thought about where you want to go? What you want to study?”

Kris shrugged, a sleepy, unconcerned expression on their face.

“If you also go to college far from home, there’s the likely possibility we’ll have a harder time doing these kinds of things, if at all…”

Kris rolled over at that, and looked Asriel in the eye. They pointed at themselves, then at him, then laced their hand with his and squeezed hard. An entire childhood of learning their gestures made it easy for Asriel to decipher them.

“You want to be with me? At my college?” He interpreted. Kris nodded resolutely.

“Heh. That sounds nice. You would be the perfect roommate, that’s for sure. Though nobody else in the dorms would ever be able to sleep again if I get to have you in my bed every night.”

Kris rolled their eyes, smirking. The current residents had about eight months to before Kris was scheduled to go to college. Plenty of time to get enough sleep before they showed up.

Human sibling and monster sibling snuggled closer, daydreaming about the possibility as they began to drift into a post-coital nap, when-

“Kids! Dinner’s ready!” Toriel announced, knocking on their bedroom. 

The lovers jerked awake and stared nervously at the door. It was shut, but not locked, since it didn’t have one. Thankfully Toriel did not try the knob.

“Wash up and come downstairs, alright?” Her voice said, fading out as she retreated away. Simultaneously, Kris and Ariel let out a breath of relief and then exchanged a humored glace. The sooner they were both out of the house, the less chance they had of getting caught. 

Graduation couldn’t come soon enough, in Kris's opinion.

Knowing Toriel would return if they didn’t show their faces downstairs with the next five minutes, Asriel got up to get properly dressed. Kris followed suit, finding their shirt and pants, but their brother stopped them before they could get their underwear back on.

“Hold it. Aren’t you forgetting something?” He asked, raising an eyebrow. Kris tipped their head to the side.

Asriel opened his hand to reveal the chastity cage once again, and Kris’ heart fluttered with thrill.

They sat back down on the bed, legs crisscrossed, while Asriel fastened the cage back in its proper place. It was a bit of a snug fit- Kris’s dick still slightly engorged after finally tasting freedom after so long, but after a bit of squeezing and squishing Asriel managed to get them secure and locked up tight.

“Don’t forget,” he said, renewing their vows. “You’re mine and only mine, now and forever.”

Kris nodded in accord, leaning forward to steal a quick kiss from his lips. 

_Same to you,_ Kris said to him with their eyes. They would be his, and he theirs, now and forever.

And if they ever broke that promise?

Well, then Asriel could just lock them up for good and throw away the key.

**Author's Note:**

> I wrote this in two days. It's been a while since I felt the words flow this easily for me. I guess the power of horniness does that to you.
> 
> For the record, it's probably not a good idea to wear a chastity cage for a hundred days straight, but this is fan fiction so who fucking cares.
> 
> Also, here's the piece of art that inspired me to write this: https://e621.net/posts/1951057


End file.
